An increasing number of applications today make use of digital video for various purposes including, for example, remote business meetings via video conferencing, high definition video entertainment, video advertisements, and sharing of user-generated videos. As technology is evolving, users have higher expectations for video quality and expect high resolution video even when transmitted over communications channels having limited bandwidth.
To permit higher quality transmission of video while limiting bandwidth consumption, a number of video compression schemes are noted including formats such as VPx, promulgated by Google Inc. of Mountain View, Calif., and H.264, a standard promulgated by ITU-T Video Coding Experts Group (VCEG) and the ISO/IEC Moving Picture Experts Group (MPEG), including present and future versions thereof. H.264 is also known as MPEG-4 Part 10 or MPEG-4 AVC (formally, ISO/IEC 14496-10).
These compression schemes may use prediction techniques to minimize the amount of data required to transmit video information by using, for example, the spatial and temporal coherences in video information.
Many prediction techniques use block based prediction and quantized block transforms. The use of block based prediction and quantized block transforms can give rise to discontinuities along block boundaries during encoding. These discontinuities can be visually disturbing and can reduce the quality of the decoded video and the effectiveness of the reference frame used as a predictor for subsequent frames. These discontinuities can be reduced by the application of a loop filter at the encoder.
A loop filter can be applied to a reconstructed frame or a portion of a reconstructed frame at an encoder. A loop filter is typically used to reduce ringing and blocking artifacts along block boundaries. Once a reconstructed frame is processed by the loop filter, it may be used as a predictor for subsequent frames. Some conventional loop filters apply different filtering strengths to different block boundaries. For example, some compression systems vary the strength of the loop filter based on how a reconstructed frame was encoded, for example, whether the block was encoded by inter-coding or intra-coding. Other compression systems apply a filter strength from a set of discrete filter strengths based on, for example, motion vector strength and the type of reference frame predictor used, such as shown by U.S. Pub. No. US-2010-0061645-A1, which is hereby incorporated by reference in its entirety.